This invention relates to High Definition Television (HDTV). More particularly, this invention relates to a method and apparatus for encoding and decoding digital data that represents video signals.
An HDTV transmission system will provide HDTV images with much greater resolution than the existing NTSC (National Television System Committee) television (TV) image. However, any HDTV transmission system is currently required to use the existing NTSC TV 6 MHz channel allocations, which are in the VHF (Very High Frequency) and UHF (Ultra High Frequency) bands. As a result, an HDTV image, which contains more picture information than an NTSC image to provide the increased resolution, must be compressed in order to fit within an NTSC TV channel. Since much more powerful video compression schemes can be implemented with digital signal processing than with analog signal processing, it is possible that some form of digital transmission system will be required for an HDTV image so that the above transmission constraint can be met. However, there has been some concern about becoming committed to an all-digital transmission system because of the potential sensitivity of digital transmission to small variations in signal-to-noise ratio at the various receiving locations. This sensitivity can result in a quick degradation in performance, which is generally not considered acceptable by the broadcast industry. By comparison, the degradation in performance for presently used NTSC TV transmission is much more graceful.
One approach to providing graceful degradation for a digital HDTV signal is to characterize the HDTV signal into classes of "more important" and "less important" information (i.e., different classes of information) and then transmit the more important and less important information using different size signal point constellations. This approach provides unequal error protection for the more important information, and allows a graceful degradation in reception quality at the TV set location because, as the bit-error rate at the receiver begins to increase with increasing distance from the broadcast transmitter, it will be the less important information of the TV signal information that will be the first affected.
Unfortunately, the amount of picture information is an HDTV signal is generally proportional to the complexity of each HDTV image that is transmitted, i.e., the amount of picture information that is considered to be more important and less important is variable. In addition, for a given HDTV transmission scheme, there is a limited channel rate (bits/symbol). As a result, the actual amount of more and less important information that can be transmitted is limited by the channel rate-- which impacts picture quality. While the general concept of separating the HDTV signal into more and less important information is an advantageous technique for providing unequal error protection, it does not give any guidance on how to allocate the HDTV signal into the different classes of information to support a variable amount of picture information within a limited capacity transmission channel--and yet maintain picture quality.